Printed circuit boards find a wide variety of uses in the electronics industry with the demand for high performance, printed wiring, or circuit boards for various applications steadily increasing. For instance, the complexity, compactness and electrical performance requirements of printed circuit boards have significantly increased over the last few years. To meet the demands of high performance along with minimum space and weight requirements, both multi-layer and rigid-flex circuits have been employed. The rigid-flex printed circuit boards typically are made by individual flexible layers and rigid layers layed-up together to form a multi-layer construction.
The flexible layers are typically an integral part of both the rigid portions and flexible portions of the printed circuit board. The rigid layers are part of only the rigid segments of the printed circuit boards. A typical material used for the flexible layers is the polyimide available under the trade designation Kapton from DuPont as the dielectric polymer film. In addition, the flexible layer typically includes an acrylic adhesive covering the Kapton in order to secure adequate bonding to the core or base dielectric and conductive layer such as copper.
The rigid segments of the rigid-flex printed circuit boards are typically formed of reinforced epoxy or reinforced polyimide laminate. In addition, acrylic adhesive is also typically used to bond the rigid and flexible layers together. Although, the acrylic adhesive exhibits excellent flexibility and heat resistance, problems have been encountered due to its high coefficient of thermal expansion as compared to copper, the dielectric core, and solder such as tin/lead solder that might be present in attaching integrated circuit chips to the board. These problems occur when the constructions are subjected to accelerated thermal cycling where the acrylic adhesive expands and contracts at a very high rate as compared to the other materials employed in fabricating the printed circuit board. The high expansion creates a great deal of stress in plated through holes and results in cracking in the walls of the through holes.
One attempt to overcome this problem is described in U.S. Pat. No. 4,800,461 to Dixon et al. This patent refers to a multi-layer rigid flex printed circuit board obtained by providing structures having rigid sections that incorporate materials which when subjected to elevated temperatures do not expand sufficiently in the Z direction to cause problems such as de-lamination and cracking of plated through holes. The flex section of these boards include flexible material that extend less than 35 mils in the rigid section and does not penetrate throughout the rigid section of the board. The insulator material of the flex segment is typically constructed of Kapton having an acrylic adhesive thereon for adhesion to the dielectric core and the small segment of overlap with the rigid segments of the board. However, such construction is not entirely satisfactory since the presence of the acrylic adhesive still remains in the flexible segments of the board and to some extent, although significantly reduced, in the rigid segments of the boards. Accordingly, the problems associated with high thermal expansion are still not entirely eliminated in such configuration.